Intersymbol interference (ISI) is a serious problem in digital communications systems. ISI occurs when a symbol, composed of one or more bits transmitted at a specified symbol rate, interferes with a subsequent symbol so that the signal for the subsequent symbol becomes distorted. ISI also occurs when a subsequent symbol interferes with a preceding symbol. The former scenario is known as post-cursor ISI because the portion of the symbol after a main pulse (also called the main cursor) is known as the post-cursor, and it is the post-cursor that interferes with the main cursor of the subsequent symbol. The latter scenario is known as pre-cursor ISI because the portion of the symbol before the main cursor, i.e., the pre-cursor, interferes with the main cursor of a preceding symbol. Serial link communications systems, such as Serializer/Deserializer (SerDes) systems, are particularly susceptible to pre-cursor ISI.
Various ways have been developed to reduce ISI at both the transmitter end and the receiver end of a communications system. Existing methods attempt to equalize transmitted data signals, with the objectives of correcting for the effects of channel attenuation and complete cancellation of ISI. For example, a conventional serial receiver consists of an analog front end that generally includes a continuous time linear equalizer (CTLE), a sampler that quantizes the analog input into digital values, a decision feedback equalizer (DFE) that uses the quantized data to adaptively feedback a correction signal to the input of the receiver, and a timing recovery unit. CTLEs and DFEs are effective at removing post-cursor ISI, but fail to adequately correct pre-cursor ISI.
Some transmitters in serial link systems implement feed-forward equalization that provides fixed, i.e., non-adaptive, post-cursor ISI cancellation. However, the transmitter located feed-forward equalizer (FFE) in these serial link systems does not provide any adaptive pre-cursor ISI correction. It is possible to implement an FFE in the receiver. In fact, some receivers include a discrete time FFE that is implemented in the analog or digital domain. Such receivers can handle both pre-cursor and post-cursor ISI, but are structurally complex and consume a large amount of power.
More advanced serial link receivers provide for both pre-cursor and post-cursor ISI correction using tap weights that are calculated through an adaptive “back-channel” equalization path, to introduce a fixed amount of equalization (pre-cursor or post-cursor) into the transmitter or to allow the receiver to adaptively control an equalizer in the transmitter. The equalizer in the transmitter is generally implemented using a finite impulse response filter (FIR) with a set of adjustable taps to change the frequency response of the filter. However, these advanced receivers require implementing pre-cursor equalization in the transmitter—which is not always possible. For example, many communications standards such as High-Definition Multimedia Interface (HDMI), DisplayPort and Universal Serial Bus (USB) do not permit the use of pre-cursor equalization in the transmitter, whether adaptive or fixed.
Accordingly, a need exists for ways to effectively reduce pre-cursor and post-cursor ISI with low cost.